Wall structures of magnetohydrodynamic conversion ducts and method of fabrication of said structures



Si -"1'1 SR June 17, 1969 o. YEROUCHALMI ETAL 3,450,905

WALL STRUCTURES OF MAGNETOHYDRODYNAMIC CONVERSION DUCTS AND METHOD OF FABRICATION OF SAID STRUCTURES Flled Feb. 20. 1967 Sheet I of 2 June 17, 1969 D. YEROUCHALMI ETAL 3,450,905

WALL STRUCTURES OF MAGNETQHYDRODYNAMIC CONVERSION DUQTS AND METHOD OF FABRICATION OF SAID STRUCTURES Filed Feb. 20, 1967 Sheet 2 of 2 United States Patent 3,450,905 WALL STRUCTURES OF MAGNETOHYDRODY- NAMIC CONVERSION DUCTS ANDMETHOD OF FABRICATION OF SAID STRUCTURES David Yerouchalmi, Issy-les-Moulineaux, and Pierre Zettwoog, Massy, France, assignors to Commissarlat a IEnergie Atomique, Paris, France Filed Feb. 20, 1967, Ser. No. 617,198 Claims priority, application France, Mar. 9, 1966,

US. Cl. 310-11 3 Claims ABSTRACT OF THE DISCLOSURE A wall structure for magnetohydrodynamic conversion ducts is constituted by an alternate arrangement of thin segments of refractory metals which form conducting electrodes and segments of refractory materials which form thermal and electric insulators, said electrodes being adapted to project inwardly from the internal surface of the duct wall.

The present invention relates to wall structures of magnetohydrodynamic (MHD) conversion ducts of the closed-loop type.

The walls of MHD conversion ducts must satisfy a certain number of conditions which can be summarized as follows:

(1) they must permit the transfer of electrons from the load circuit to the plasma;

(2) they must aflord resistance to chemical corrosion and must not contaminate the plasma as a result of degassing;

(3) they must be at a high temperature in order to ensure that thermal losses are negligible;

(4) they must ensure a certain distribution of electric potentials.

The conditions summarized above are exacting and often contradictory. The transfer of electrons from the load circuit to the plasma by virtue of the' thermionic emission of electrodes usually takes place when the electrodes are fabricated of refractory metal and when the temperature is either higher than or equal to 1200 K.

The resistance of a material to chemical attack and its properties of non-contamination of the plasma are greater as the temperature of the material is lower, which runs counter to the need for minimized thermal losses and good thermionic emission.

The distribution of electric potentials as defined by the series of electrodes and insulators can be impaired by short-circuits which take place behind the electrodes, in particular across ceramic materials which have been rendered conductive either by deposition or by migration of cesium vapors, or even by condensed liquid cesium.

The present invention is concerned with Wall structures of MHD conversion generators which meet the conditions mentioned above and which possess in particular the following properties:

Thermionic emission which is always suflicient;

Low temperatures of insulating ceramic materials, only the electrodes of refractory metal being permitted to attain the temperature of the gas;

Very low thermal losses;

Total freedom from short-circuiting of electrodes.

The Wall structures of MHD conversion ducts in accordance with the invention are characterized in that they are constituted by an alternate arrangement of thin segments of refractory metals forming the electrodes and segments of refractory materials which serve as thermal Patented June 17, 1969 and electric insulating material, said electrodes being adapted to project inwardly from the internal wall of the duct.

By virtue of their low thermal capacity, the electrode segments attain their working temperature at a fast rate. The process of heat transfer by convection at high velocity from the hot plasma to the insulating walls takes place either by conduction through the segments which is negligible owing to the small thickness of said segments or by convection at low velocity or conduction within the spaces filled with gases which are trapped between each segment, this heat transfer process being also of low efficiency if the segments are set fairly close together. The main temperature difference between ambient temperature and the plasma is therefore within the layer which is immediately adjacent to the insulating refractories, the thermal flux being of a low order and the ceramic elements being at low temperatures.

A better understanding of the invention will be gained by consideration of the following description, reference being made to the accompanying drawings, in which:

FIG. 1 shows diagrammatically a portion of a MHD duct in accordance with the invention;

FIG. 2 is a longitudinal sectional view of a MHD conversion duct;

FIGS. 3 and 4 are two transverse sectional views of a MHD conversion duct, the view of FIG. 3 being taken in cross-section along the line I-I of FIG. 1;

FIGS. 5, 6 and 7 illustrate the assembly of the different elements of a duct in accordance with the invention.

As shown in FIG. 1, the electrodes which are designated by the references E E E and E are fixed in a transverse section of a duct which is fabricated of ceramic insulating material. The electrodes referred to are formed of a portion of greater thickness (a) which is embedded in the mass of ceramic material, said portion (a) being brazed to a portion (b) of smaller thickness which projects from the internal surface of the duct wall up to a height (h). The ratio of the height (h) to the distance (d) between the electrodes is approximately 1:5.

The MHD conversion duct is fabricated of a refractory material such as alumina, beryllium oxide or boron nitride and the electrodes are fabricated of a refractory material such as tantalum, nickel or of a noble metal such as platinum.

The duct may have any desired cross-sectional profile without thereby affecting the properties claimed. In the remainder of the specification, reference will more especially be made to a duct of circular cross-sectional configuration, although it will be understood that the invention also applies to ducts having other cross-sectional shapes.

The invention is also concerned with a method of fabrication of these ducts and comprises two modes of execution.

In a first mode of execution, a tube fabricated for example, of alumina, is cut lengthwise along a chord of the circular cross-section which projects by approximately 10% beyond the diameter-line of said cross-section as shown in FIGS. 3 and 4. The same operation is repeated in the case of a second tube having the same length and the same circular cross-section. In order to construct the conversion duct, only those two portions of tube 1 and 2 whose residual cross-section is greater than a semi-circle are cut so as to form a suitable scarf joint (as shown in FIG. 7) and to form by assembly a cylinder having a perfectly circular cross-section. Such pairs of elements can be joined lengthwise in interfitting relation with pairs of elements having the same dimensions so as to constitute ducts of the required length. As is thus shown in FIG. 5, two pairs of elements 1, 2 and 1, 2 can accordingly be fitted one inside the other and FIG. 6

shows the mode of assembly of the elements 2 and 2'. The complete structural assembly thus formed is housed within a concentric tube fabricated, for example, of aluminum oxide, thereby centering the MHD conversion duct and ensuring leak-tightness.

The semi-cylindrical tubes are provided internally with equidistant grooves 3 of small width and depth for the purpose of accommodating annular metal segments 4 which constitute the electrodes. The grooves referred to are formed, for example, by means of a diamond-tipped saw. In different examples of construction, the annular segments had a thickness varying between 0.25 mm. and 0.5 mm. and a depth of the order of 0.5 mm. and were separated by 0.5 mm. of ceramic material.

According to one of the important features of the invention, the circular metallic electrodes have an internal diameter which is slightly smaller than that of the ceramic rings, This arrangement permits a rapid temperature rise of the metal relatively to the ceramic material which remains at a relatively low temperature ranging from 500 to 1000 K. approximately. This is due to the presence of a gaseous zone of low turbulence between two electrodes such as E and E with the result that very little heat is transferred to the ceramic material; in addition, both the mass and the insulating properties of the ceramic material constitute a very elfective thermal barrier.

The second mode of execution of the invention is more especially applicable to the case in which the thermal masses of the electrodes are very small with respect to the ceramic insulators which surround these latter. In accordance with this mode of construction, a ceramic tube is employed as a starting element and cut with a diamond-tipped saw. The tube is then re-assembled by placing thin annular segments 4 of refractory metal between the ceramic rings 5, as shown in FIG, 2. The

ceramic rings have a thickness, for example, of 0.25 to l being lower than that of the ceramic material constituting the tube, then sintered at a temperature of the order of 1600 C.

During this operation, a ceramic-to-metal bond is formed by brazing. After cooling, a monolithic structure isobtained and has good mechanical properties.

After internal and external grinding, the completed assembly is fitted within a concentric tube of refractory material which ensures leak-tightness and centering of the MHD conversion duct while at the same time preventing any short-circuits within the duct support structure.

What we claim is:

1. Wall structures for magnetohydrodynamic conversion ducts comprising an alternate arrangement of thin segments of refractory metals forming the electrodes and segments of refractory thermal and electric insulation material, said electrodes projecting inwardly from the internal wall of the duct, the ratio of the internal projection of said electrode to the distance between adjacent ones of said electrodes being approximately 1:5.

2. In a method of fabrication of walls for magnetohydrodynamic conversion ducts, the steps of assembling semi-cylindrical tubes of ceramic material in which equidistant grooves have been cut internally in order to accommodate annular metal segments which serve as electrodes.

.3. In a method of fabrication of walls for magnetohydrodynamic conversion ducts, the steps of cutting a ceramic tube into rings, re-assembling said rings by placing thin annular segments of refractory metal therebetween, clamping the complete assembly within a support structure of carbon silicide, and sintering at a temperature of the order of 1600 C. to form a ceramic-to-metal bond; by brazing.

References Cited UNITED STATES PATENTS 3,154,702 10/1964 Rosa 3l0--11 3,397,331 8/1968 Burkhard 31011 DAVID X. SLINEY, Primary Examiner. 

